linux-kernel - Re: [PATCH -tip 0/2] Temporary RCU fixes for notrace and hotplug CPU

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Date:	Tue, 25 Aug 2009 10:11:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...e.hu>, Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org, dipankar@...ibm.com,
	akpm@...ux-foundation.org, josht@...ux.vnet.ibm.com,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH -tip 0/2] Temporary RCU fixes for notrace and hotplug CPU

On Tue, Aug 25, 2009 at 12:25:49PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > On Tue, Aug 25, 2009 at 10:00:47AM +0200, Ingo Molnar wrote:
> > > 
> > > btw., i'm still seeing crashes with the latest RCU bits:
> > > 
> > > [   20.621740] Testing event sys_enter_futex: OK
> > > [   20.629738] Testing event sys_exit_futex: OK
> > > [   20.637737] Testing event lock_acquire: [reboot]
> > > 
> > > Possibly due to infinite recursion as well. Config attached.
> > 
> > Color me confused...
> > 
> > Unless someone has a better idea, I will send in a patch that adds
> > "notrace" to every RCU API member used by any file in the kernel
> > that has "trace" in its name (excluding ptrace.c and rcutree_trace.c,
> > of course).  This list is as follows:
> > 
> > 	call_rcu()
> > 	call_rcu_sched()
> > 	rcu_read_lock()
> > 	rcu_read_unlock()
> > 
> > So, any better ideas?
> 
> Tracers using RCU should use the _notrace() version of read_lock/unlock.
> I think the callers should be fixed rather than RCU.
> 
> Tracepoints have been designed to use the _notrace variant on the
> instrumentation site. The core of tracepoint management use
> call_rcu_sched(), which can be traced without any problem.
> 
> I have not followed the late tracing development as closely though, so
> errors might have crept in.

Or I might have inadvertently broken something in a non-obvious (to me,
anyway) manner.

So, would you be willing to look at commit bc33f24bd in the -tip
tree and see if there is anything I broke other than the now-fixed
rcu_read_lock_sched_notrace() and rcu_read_unlock_sched_notrace()?
And for that matter, whether my alleged fix for these two API members
really does fix the problem (-tip commit 7c614d6461)?

The -tip tree is at:

	git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git

And these patches are on the tip/core/rcu branch.

							Thanx, Paul
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